1. Field of the Invention
The present invention relates to an X-ray irradiation apparatus such as an X-ray microscope, an X-ray analysis device, an X-ray exposure apparatus or the like, and more particularly, to an X-ray generating device in an X-ray irradiation apparatus.
2. Discussion of the Related Art
Laser plasma X-ray sources (or "LPX's") serve X-ray generating devices by focusing pulsed laser light on a target material placed inside a vacuum vessel so that the target material is converted into a plasma, and X-rays are generated from the plasma. Such LPX's have small sizes while having brightnesses comparable to those of undulators. As a result, such X-ray sources have attracted attention in recent years as light sources for X-ray equipment such as X-ray analysis devices and X-ray exposure apparatus, etc.
Generally, in X-ray irradiation systems using LPX's, X-rays radiated from the LPX are directed onto the object of irradiation, such as a sample in an X-ray microscope or X-ray analysis device or a mask in an X-ray exposure apparatus, using an illumination optical system. The transmitted or reflected light is then focused on a detector or object of exposure, such as a substrate coated with a resist, by an image-focusing optical system. In conventional X-ray optical systems, once the alignment of the X-ray light source position and X-ray optical elements has been completed, no further adjustment is performed. However, long-term operation of LPX's introduces a number of difficulties that must be addressed.
First, the position of the X-ray source should not fluctuate from a specified position. Fluctuation in the position of X-ray generation may occur when the target material is replaced. Furthermore, fluctuation in the X-ray generation position over time occurs as a result of variations in the beam divergence and direction of emission of the laser light caused by changes in the temperature of the laser rod and changes in the temperature of the crystal used for the generation of second harmonics of the laser light. In addition, the position of X-ray generation may also fluctuate as a result of variations in the target position caused by deformation of the target holding member as a result of stray particles emitted in the vicinity of plasma generation.
In an X-ray microscope, if the X-ray light source position fluctuates greatly, the sample illumination also shifts so that the area to be observed is no longer in the field of view. Furthermore, in the case of multiple exposures, even a slight fluctuation in the position of the X-ray source causes the sample illumination position to spread. Consequently, the X-ray dosage per unit area of the sample is reduced as compared to cases having no fluctuation in the position of the light source, thereby increasing the amount of required X-ray irradiation.
In an X-ray exposure apparatus, if the X-ray light source position shifts over time, the position on the mask illuminated by the X-rays also shifts. As a result, IC patterns cannot be accurately transferred.
Second, to achieve stable long-term operation, a continuous supply of target material must be provided. It is also preferable to be able to recover and reutilize the target material, especially if the target material is expensive. That is, the recovery and reutilization of the target material can lower the running costs of the apparatus. Therefore, target materials are used in the form of gases, liquids or fine solid particles. Target materials in such forms are fed out into the vacuum vessel from a target material feed-out portion, such as a nozzle, and then irradiated with laser light to generate X-rays.
Conventional methods use a substance that is a gas at room temperatures as the target material (for example, nitrogen, carbon dioxide, krypton or xenon). The target material is sprayed or jetted from a nozzle after which the target material gas or cluster form material resulting from the adiabatic expansion of the gas is irradiated with laser light. Since the target material is a gas at room temperatures, there is no deposition of this target material on optical elements. As a result, a gaseous target material is favorable because no deterioration occurs in the performance of the optical elements.
The gas sprayed from the nozzle expands inside the vacuum vessel as a result of free expansion. Consequently, the density of the gas decreases abruptly as the distance from the nozzle increases. Accordingly, if it is desired to increase the quantity of X-rays emitted from the plasma, it is necessary to generate the plasma in the vicinity of the nozzle where the density of the gas (or cluster form material) is large (i.e., within a distance of several tenths of a mm to several mm from the tip end of the nozzle).
If a liquid is used as the target material, the liquid is continuously sprayed from the nozzle or sprayed from the nozzle in the form of liquid droplets, and these droplets are irradiated with laser light. If fine solid particles are used as the target material, the fine solid particles are sprayed together with a gas. These fine particles are then irradiated with laser light. Also, irradiation with laser light is performed in the vicinity of the nozzle, where the positional stability of the target material is high.
Moreover, if X-rays are generated over a long period of time using the conventional X-ray generating device with a gas target material, a drop in X-ray intensity will be encountered. Furthermore, with long-term X-ray generation using a conventional X-ray generating device having a solid or liquid target material, the X-ray intensity becomes unstable.